In many film applications, such as packaging of foodstuffs, chemical and hazardous materials and in medical applications, the industry requires films having certain properties. In the packaging of foodstuffs, for example, the films must have high puncture resistance, high clarity and gloss, reduced permeability to gases and/or vapors and good weld strength. The films used to manufacture containers for chemicals and hazardous materials must have a high puncture resistance, high elongation strength, high tear resistance, chemical resistance and good weld strength. Films used in medical applications, such as blood bags, must have a high puncture resistance, low modulus, high tear resistance, autoclavability and good weld strength.
Films made from ethylene polymers, e.g., HDPE and LLDPE, and propylene polymers, such as crystalline homopolymers of propylene and random copolymers of propylene and ethylene do not provide such a combination of desirable properties.
Crystalline random copolymers of propylene and ethylene are frequently used to prepare such films because of the inherent clarity of such copolymers. However, films prepared from crystalline copolymers of propylene and less than 3% ethylene or blends thereof with other olefin polymers do not provide good seals or weld strength since the melting point of these copolymers or blends thereof is too close to the melting point of the base polymer layer they are laminated to or coextruded with, which is typically a crystalline homopolymer of propylene. Further the resulting laminate or coextruded material lacks dimensional integrity or stability.
When films are prepared from crystalline random copolymers of propylene and ethylene having about 5-6% ethylene, the melting point of said copolymer is decreased with respect to the melting point of the crystalline homopolymers of propylene so that such a copolymer film does have certain weldability properties. However, the insertion of the comonomer disrupts the crystalline structure with the concomitant formation of an increased fraction of xylene soluble polymer at room temperature. Consequently, the mechanical properties of films formed from such a copolymer are adversely affected. Furthermore, the presence of a high xylene soluble polymer fraction renders the film material susceptible to attack from organic substances and, therefore, unsuited for use in food packaging. Films prepared from blends of such crystalline copolymers of propylene with other polymers do not overcome these disadvantages.
Therefore, there is a need for a film or sheet material having a low seal initiation temperature as well as other desirable properties comprising a propylene polymer material having a low xylene soluble polymer fraction at room temperature and weldability.
All parts and percentages used in this application are by weight unless otherwise specified. Ambient or room temperature is approximately 25.degree. C.